数字组合

回溯：

//
// Created by jt on 2020/9/24.
//
#include <vector>
using namespace std;

class Solution {
public:
    /**
     *
     * @param n int整型
     * @param k int整型
     * @return int整型vector<vector<>>
     */
    vector<vector<int> > combine(int n, int k) {
        // write code here
        vector<vector<int>> res;
        vector<int> vec;
        backtrack(res, vec, n, k);
        return res;
    }
    void backtrack(vector<vector<int>> &res, vector<int> &vec,
            int n, int k) {
        if (vec.size() == k) { res.push_back(vec); return; }
        int start = vec.empty() ? 1 : vec[vec.size() - 1] + 1;
        for (int i = start; i <= n; ++i) {
            vec.push_back(i);
            backtrack(res, vec, n, k);
            vec.pop_back();
        }
    }
};